Allotted is the past form of allot, and also the adjective: it describes the share that has already been given. Your allotted time is the time set aside for you. An allotted seat is the one assigned to you. “We stayed within the allotted budget” means we did not spend more than the share we were given. It has the same slightly formal, planning feel as allot. Though actually, the word looks back: the dividing has already happened. You are talking about the piece that was set, not about the act of setting it right now.